Optical system for television receivers



F. OKOLIC SANYI OPTICAL SYSTEM FOR TELEVISION R ECEIVERS Sept. 29, 1942 2 Sheets-Sheet 1- Filed April 29, 1940 S i nenc 01:0 a a8, M. MA

Patented Sept. 29, 1942 search Room OPTICAL SYSTEM FOR TELEVISION RECEIVERS Ferenc Okolicsanyi, lliensington, London, Engand Application April 29, 1940, Serial No. 332.379 In Great Britain May 1, 1939 5 Claims.

The present invention relates to television receivers of the type employing a cathode ray tube and optical means for forming on a projection screen an enlarged image of the picture reproduced on the image screen by the cathode ray tube.

The invention also relates to an improved form of such optical means.

According to the invention, there is provided a cathode ray tube television receiver which is,

adapted to produce on the image screen of the cathode ray tube a picture havingits dimension contracted in the frame scanning directionfand which is provided with an optical projection system comprising two crossed cylindrical optical elements, which system has such a greater magnifying power in the frame scanning direction than in the line scanning direction that in the final projected picture the frame dimension is restored to its correct magnitude.

According to the invention in another aspect there is provided an optical projection system for use in conjunction with a cathode ray tube television receiver the said system comprising two crossed cylindrical optical elements, and being such that, when, placed in the operative position, its greater magnifying power is in the frame scanning direction.

An optical system of this type has considerable advantages over the more usual spherical pro- J'ection lens. The separate cylindrical members can be made of large size and large optical aperture and can yet be corrected without the cost becoming prohibitive. The large size of these elements enables them to have a long focal length for a given aperture, thus enabling a given picture size to be obtained with a long throw. This is of importance when the apparatus is used in a cinema, since it enables the apparatus to be installed in the operators booth instead of in the auditorium. Also, the large size of the optical elements enables a cathode ray tube of large size to be employed instead of the usual small tubes used for projection r irposes, thus increasing the available light flux.

In a preferred embodiment of the invention shown in Fig. 1, the cylindrical elements comprise cylindrical mirrors l and 2 formed of silvered sheets la and 2a of Celluloid or the like,

secured to bearing members lb and 211 out to the required curve. In this way. corrected elements of any size can easily be obtained.

The mirror I, curved in the frame scaning direction faces the fluorescent screen 3 of the cathode ray tube 4 and reflects the light therefrom on to the second and much larger mirror 2 which surrounds the end of the cathode ray tube 4 and faces the projection screen 5. This second mirror 2 is curved in the line scanning direction, and its silvering is interrupted in the centre, leaving a transparent window 6 through which the light from the fluorescent screen 3 reaches the first mirror I. Owing to the smaller optical distance between the first mirror I and the fluorescent screen 3, the magnification in the frame scanning direction is greater than in the line scanning direction. The necessary contraction of the picture 1 on the fluorescent screen in the frame scanning direction is obtained by adjusting the appropriate time-base circuit 8 to give a smaller sweep in this direction. For this purpose no fundamental change from the normal circuit is required. All that is necessary is to adjust the amplitude of the output in any suitable manner.

The television receiver signal separator and line scanning time base circuit indicated at 8 is of the usual kind.

In the example shown, the picture on the cathode ray tube is 10 inches x 4 inches. The mirror I is 4 feet from the fluorescent screen 3, and the distance between the latter and the projection screen 5 is 150 feet. The focal length of the mirror I is given by the expression 1 1 1 Fi m and that of the mirror 2 by 1 1 1 Tf fi The mirror I may be approximately 2 feet square and the mirror 2 approximately 4 feet square.

When using a cathode ray tube for projection purposes it is customary to employ a special electron-optical focussing system to give a spot which is somewhat larger in the frame scanning direction than in th line scanning direction, in order to avoid the appearance of strips in the projected image. When employing the present invention, this result is achieved with the use of a normal spot, owing to the contraction of the picture in the frame scanning direction.

This may be further explained with reference to Figs. 2 and 3. In Fig. 2 is shown an 8" x 10" frame. At II is shown scanning by a circular spot which is exactly the width of one scanning line. This gives the full transmitted definition, but it is found that the raster is visible, and that inter-line flicker is noticeable. To get over this difdculty, it is customary to make the spot cover two lines, as shown at II. This removes the disadvantages mentioned above, but unnecessarily reduced the definition in the line scanning direction. At (3 is shown a further proposal, in which the scanning spot is made elongated in the frame scanning direction, thus removing the disadvantages of the spot in I I, and keeping full definition in the line scanning direction which was lost in [2. However a further disadvantage arises in the method l3, in that the electron-optical focussing system needed to produce this spot are complicated and difiicult to design.

Fig. 3 shows an arrangement in accordance with the present invention. The spot size is kept as in II, but the picture is reduced to 8" x 10", i. e. is halved in the frame scanning direction.

In the arrangement of Fig. l the mirrors may be of the kind known as Mangin mirrors which are corrected for chromatic and spherical aberrations. This is done by having the reflecting surface of the mirror on the back surface of a sheet of transparent material which has a different curvative on the front surface from the back surface. Such a mirror is shown in Fig. 4, where I6 is a mirror surface on a sheet of material 18 whose front surface I! has a different curvature from the rear mirror surface Hi. The direction of the incident light is shown by the arrow I9. The material l8 acts as a lens or prism, and by suitable design of the radii of curvature of the two surfaces I6 and I1 optical corrections may be achieved.

I claim:

1. A cathode ray tube television receiver adapted to produce on the image screen of the cathode ray tube a picture having its dimension contracted in the frame scanning direction, and an optical projection system comprising a first cylindrical mirror having power in the frame scanning direction and facing said image screen, a second cylindrical mirror surrounding said image screen and having power in the line scanning direction and adapted to receive light from said first screen and to project it on to a projection screen, said system having such a greater magnifying power in the frame scanning direction than in the line scanning direction that in the final projected picture the frame dimension is restored to its correct magnitude.

2. A cathode ray tube television receiver according to claim 1 wherein said mirrors comprise backing members cut to the required curve, and silvered sheets of pliable material secured thereto.

3. A cathode ray tube television receiver adapted to produce on the image screen of the cathode ray tube a picture having its dimension in the frame scanning direction reduced to one half relative to the dimension in the line scanning direction, and an optical projection system comprising a first cylindrical mirror having power in the frame scanning direction and facing said image screen, a second cylindrical mirror surrounding said image screen and having power in the line scanning direction and adapted to receive light from said first screen and to project it on to a projection screen, whereby the doubly great magnifying power of said system in the frame scanning direction gives the final projected picturethe correct magnitude in this direction.

4. A receiver as claimed in claim 1 wherein said mirrors are of the Mangin type of corrected mirrors.

5. A television receiver comprising a cathode ray tube, means for producing on said tube a television image having its dimension contracted in the frame scanning direction relative to the dimension in the line scanning direction, and an optical system for projecting an image of the screen of said tube on to a receiving screen, said optical system comprising a first cylindrical mirror having focussing power in the frame scanning direction and facing said cathode ray tube for receiving light from the screen thereof, and a second cylindrical mirror having power in the line scanning direction and positioned to face said first cylindrical mirror and said receiving screen, and having an aperture therein corresponding to said cathode ray tube.

FERENC OKOLICSANYI. 

